Phthalic anhydride (PAN) is an important intermediate chemical in the chemical industry. One important use is in the production of alkyl phthalates such as di-isononyl or di-isodecyl phthalates which are used as plasticisers typically for polyvinyl chloride. These phthalates may be further hydrogenated to the corresponding di-cyclohexanoates. Phthalic anhydride has been produced on an industrial scale for many years. Phthalic anhydride is typically produced on a commercial scale by the vapour phase oxidation of primarily ortho-xylene (o-xylene), or less frequently of naphthalene, over a heterogeneous metal oxide catalyst. Typically air is used as the oxidant and the process generally uses a vanadium oxide catalyst, more specifically vanadium pentoxide on a support.
After the reaction, the reaction product vapour mixture containing the crude phthalic anhydride that has been produced passes to a cooling stage where it is cooled, generally by a gas cooler, and is subsequently passed to optionally a liquid condenser and finally to a switch condenser for condensing the PAN. Finally the condensed phthalic anhydride is subjected to a purification or finishing step. Phthalic anhydride processes are disclosed in more detail in WO 2009/040245 and WO 2009/040246.
The efficiency of a phthalic anhydride plant is measured in terms of the number of grams of ortho-xylene that can be processed for each normal cubic meter of oxygen-containing gas or air that is fed to the raw material section (known as the loading). The greater the amount of ortho-xylene per unit of gas flow, the greater is the efficiency of the facility. Considerable attempts have been made over the years to increase the loading, and loadings above 80 gram/Nm3 of ortho-xylene in air have been reported.
The oxidation reaction is highly exothermic. The process typically operates with reaction mixtures of the vaporised organic material in air, at temperatures higher than 300° C., and the mixtures have compositions that are typically inside the explosive range, and this generally all through from reactor feed to effluent. The reaction conditions need to be controlled very tightly in order to minimize the occurrence of local excessive exotherms, which can cause the reaction mixture to detonate. The reaction is most commonly performed in a tubular reactor, i.e. a reactor designed as a tube-and-shell heat exchanger, with the catalyst located as a fixed bed of particles inside the tubes, and a molten salt bath circulating on the shell side for removal of the reaction heat. The reactor tubes typically have a length of at least 3 meters, and a typical internal diameter of about 25.4 mm (1 inch). The reactor feed typically enters the reactor tube at the top and flows down towards the bottom.
In order to further improve process stability, and save energy in compressing the air for the reaction, the reaction pressure is preferably kept low, which means that a low pressure drop is desired over the catalyst bed itself and over the equipment downstream thereof, such as over the phthalic anhydride condensers. In order to provide a low pressure drop over the catalyst bed, commercial catalysts have since decades adopted a ring-type or hollow cylinder design, whereby the active catalyst is coated as a thin active layer onto the (inner and) outer surface of a ring-type inert support, usually of ceramic material. The preferred catalyst is composed of a mixture of vanadium pentoxide, titanium dioxide, and several other metal, alkali and earth-alkali components in varying concentrations, typically coated on a ceramic ring or hollow cylinder material. Such a hollow cylinder may e.g. have 7 mm as the outer diameter (OD) and 4 mm as the inner diameter (ID), and have a height (H) of 7 mm. Alternatively, the cylinder may have 8×6×5 mm as (OD×H×ID) dimensions.
The active layer coating typically contains an organic binder and/or adhesive to help in keeping the layer in place on the surface of the inert support. During the initial startup and the subsequent operation of the catalyst at the typical operating conditions, the organic binder and/or adhesive typically disintegrate and disappear. The catalytically active material remains in place as a thin and fragile layer, which may rapidly fall apart into a powder form upon exposure to mechanical action and/or upon exposure to a liquid when it may readily form a slurry.
A commercial phthalic anhydride process may typically employ more than 10,000 vertically mounted tubes per reactor, and the flow of the reaction mixture needs to be well distributed over the many reactor tubes. This reduces the risk for local temperature excursions, and by which process stability improves. It also reduces differences in the conversion levels reached over the individual tubes, such that the reactor may be operated closer to the desired conversion level and product quality problems, because of byproduct formation, are reduced. Because of the low pressure drop available during operation over the oxidation reactor, this requires a close similarity in the composition and structure of the catalyst bed in each of the reactor tubes.
The catalyst slowly looses activity through its use, and typically the salt bath temperature, and thereby the reaction temperature, is then carefully adjusted upwards to compensate for the activity loss. This may be done up to a level where side reactions and byproduct formation have become excessive, at which point the catalyst is considered at the end of its life. Usually after several years in operation, such as after 3 or 4 years, the now spent catalyst needs to be removed and replaced by catalyst having a higher activity and/or selectivity, typically with fresh catalyst.
The vanadium in the spent catalyst is highly valuable, and is typically recovered and reused. Also the inert support represents sufficient value, such that its recovery and reuse is of high interest. A good separation is therefore important of the active material containing the vanadium from the inert support. The active material is typically recovered as a slurry of active material powder in a liquid phase, usually by washing the spent catalyst with water, and it is important for an efficient recovery and recycling of the vanadium metal that contamination of the slurry with inert powder, such as with dust originating from the inert support, is minimised. The reuse of the inert support is also improved if physical damage to the support particles, in particular during the removal of the spent catalyst from the reactor, is minimised.
In order for the new catalyst bed to be loaded correctly and similar to the other reactor tubes, it is important that the tubes of the reactor are empty and clean prior to the loading. WO 2006/131557 discloses a method for controlling the unloading of the catalyst from a tubular reactor by using one or more light sources.
Unloading the catalyst from the bottom of a tubular reactor has several problems, in particular when the catalyst is not free-flowing and needs to be dislodged in order to release from bridging between particles or from the tube wall, such as described in U.S. Pat. No. 4,411,705. An alternative is to have personnel enter the bottom section of the reactor, below the bottom tubesheet, and after having removed the support for the catalyst bed, poke the underside of the catalyst bed with a metal wire to dislodge the catalyst particles and have them fall from the bottom of the tube.
WO 2006/131556 discloses that an incorrectly filled tube, in a tubular reactor such as in a phthalic anhydride process, must be identified from the bottom tubesheet in order to allow emptying the tube from the bottom.
It is typically impractical or impossible to remove the bottom head of the reactor, so the space below the bottom tubesheet of the tubular reactor is typically a confined space and only accessible through a manhole. During catalyst unloading from the bottom, any personnel accessing the bottom tubesheet, to for instance remove the bed support, may come in contact with a stream of falling catalyst particles. In addition, the particles may be accompanied by dust, and more dust may be formed when the catalyst pellets fall onto a solid surface. Dust is a problem of industrial hygiene, and the catalyst dust may be particularly problematic because of its possible toxicity, a.o. because of the vanadium content. The bottom space of the reactor during unloading of the catalyst from the bottom therefore becomes an inhospitable confined space of limited dimensions, wherein personnel typically needs to wear personnel protection equipment such as a breathing apparatus.
In order to overcome the need for personnel to enter the inhospitable space inside a reactor, U.S. Pat. No. 4,411,705 discloses the dislodging of used catalyst inside the tubes of a tubular reactor in the petrochemical industry by means of a string or burst of missiles, from the bottom of the reactor until all of the catalyst has fallen from a tube. At the same time a gas may be caused to flow down the tube to induce the particles to flow down the tube. The falling used catalyst particles are preferably collected in a particle collector under the tube, from which they may be removed by attaching a vacuum tube.
This use of missiles from the bottom to dislodge the catalyst particles and have them fall out the bottom of the tube is a complex operation, for instance because it needs to be assured that all tubes have been treated by the missile gun and have successfully been emptied. When launching the missiles from the bottom of the tube, it is difficult to at the same time also collect the fallen particles and the dust from underneath the bottom tubesheet.
Because of the complexity of the method of unloading the catalyst from the bottom, it is preferred to unload the catalyst from the top. The catalyst particles may be vacuumed out from each of the tubes by entering the reactor tube with a smaller vacuum tube, through which the catalyst particles are then vacuumed up.
The particles of the catalyst often become bridged through their use in service. They typically need to be dislodged before they may be vacuumed out. Dislodging the particles during the vacuuming is conveniently done by mechanical action with the vacuum tube, provided the vacuum tube is made from a rigid construction material.
The vacuum tube is usually made longer than the reactor tube, such that it is able to reach all the way from above the top of the reactor tube down to the bottom of the tube. Because of weight and ease of handling, the vacuum tube is therefore conveniently made from a light weight material, typically from rigid PVC.
We have now found that the vacuuming out of catalyst from the tubes of a phthalic anhydride reactor using a rigid PVC vacuum tube still poses problems. For exchanging the catalyst, the reactor top head is usually removed and a tent or cabin is mounted covering the top tubesheet of the reactor, in order to protect the personnel performing the catalyst exchange, and the material and equipment they are handling, from adverse weather conditions. With the rigid PVC vacuum tube, the inner height of the tent or cabin above the top reactor tubesheet needs to be longer than the length of the vacuum tube, allowing sufficient manoeuvring with the vacuum tube. The total height of the tent or cabin becomes large, which puts extra requirements on its construction, for instance because of wind exposure.
Also, by the mechanical action with the vacuum tube to dislodge the catalyst particles, pieces are chipped off from the tip of the vacuum tube, and the vacuum tube mouth looses its integrity. This makes the vacuuming less effective and increases the time required for emptying the reactor, and thus for the catalyst change out. A longer catalyst change out is at the expense of time on stream for the reactor, and thus of plant capacity. The reduced efficiency of the vacuuming reduces the effectiveness of the dust removal during the vacuuming and increases the time during which personnel may become exposed to catalyst dust.
A deformed vacuum tube mount also increases the risk to leave catalyst particles and/or scale sticking to the reactor tube wall, which if not removed before loading the new catalyst, may cause higher pressure drop over some of the tubes and/or local channelling of the flow of the reaction mixture, which creates an inherent risk for process instabilities.
The vacuum tube also becomes shorter and needs to be replaced when it has become too short to reach the bottom of the reactor tube. This is usually noticed when the reactor tube still contains catalyst and catalyst dust, and such replacement of a vacuum tube further increases the risk for undesired exposure of the personnel to catalyst dust. In addition, the pieces of rigid PVC breaking from the vacuum tube are vacuumed up with the catalyst particles. The collected used catalyst becomes contaminated and this makes the recovery of the metals from the used catalyst, and of the inert catalyst support, more difficult.
WO 93/00158 discloses a method for catalyst unloading of tubular reactors, whereby an flexible air lance is introduced into the reactor tube and high pressure gas is injected through a jet to dislodge the catalyst and to blow the fluidized catalyst out of the top of the tube, where a high volume vacuum source creates a negative pressure in a plenum chamber located on top of the tube, through which the catalyst is removed. WO 93/00158 describes this method to be applicable to reactors used in a variety of processes, including in the production of phthalic anhydride.
WO 98/02239 describes a similar method for emptying a tube reactor, such as ethylene oxide, acrylic acid or terephthalic acid reactors. In this method, the bottom end of the reactor tube to be emptied is first temporarily sealed, after which from the other end a flexible pressure pipe is introduced into the reactor tube and gas under pressure is injected through a nozzle in order to detach the catalyst particles, which are then sucked up by a suction pipe which is connected to the top of the reactor tube to be emptied.
In these methods, the use of high pressure gas injection and of seals at the connections of the vacuum pipe to the reactor tube and around the pressure pipe or lance cause significant risks for catalyst dust to escape, which is a safety and industrial hygiene problem for the workers involved in the catalyst removal.
U.S. Pat. No. 4,568,029 discloses a process for unloading catalysts from multi-tube reactors. The process can employ steel rods, gravity and air jets. No hose together with a vacuum is applied.
US 2005/0109377 discloses the removal of catalyst from the tubes of a tube bundle heat exchanger by inserting a rotating drill driven by a drilling machine into the tube, the drill having a steel shaft and a drill tip provided with teeth made of stellite, and using a rotation rate of 220 to 280 rpm. This method is proposed for the cleaning of tubes in which the catalyst solids are no longer present in loose particulate form, but rather at solid blocks, and/or are adhering particularly firmly to the inner walls of the tubes. US 2005/0109377 proposes this method for a variety of heterogeneously catalyzed partial oxidation processes, such as the conversion of o-xylene or naphthalene to phthalic anhydride. This method however causes excessive physical damage to the catalyst particles used in the production of PAN, such that the inert support recovered from the spent catalyst cannot be recycled. In the comparative example of US 2005/0109377, an attempt to suck out the tubes of a reactor by means of a suction tube which consisted of a plastics hose having, mounted at the tip, an 80 cm-long metal tube cut obliquely and having 85% of the reaction tube diameter, was unsuccessful.
There therefore remains a need for a method to remove the spent catalyst from a phthalic anhydride process that allows recovery and reuse of the vanadium and of the inert support particles.
EP 1226865 A2 discloses the removal of catalyst from a shell-and-tube reactor used in a many catalytic reactions. In this method, an aspirating tube, connected to an exhaust gas aspirator, is inserted from the top into a reaction tube in order to remove the catalyst together with a stream of air. The aspirating tube may have high rigidity and may be difficult to deform, or it may have flexibility and can be bent. EP 1226865 discloses that an aspirating tube made of polyethylene has good workability and can easily be used because of properly bending. In a variant of the method, the kind of material or shaped structure at the tip portion of the aspirating tube can be different from that of the rear portion. The use of an aspirating tube consisting of a polyethylene tube is exemplified in EP 1226865 for extracting catalyst from a process for producing methacrylic acid from methacrolein, and from a process for producing acrylic acid from propylene. In one example the aspirating tube was provided, to the side where the aspirating tube for extraction was inserted to the reaction tube, with a stainless steel adaptor which was cut to form an end surface having a hollow portion.
We have also found that when using a vacuum hose made of rigid PVC, a static electricity charge tends to build up on the vacuum hose, which may transfer to the personnel handling the vacuum hose. The static electricity charge accumulated in the person may then discharge, and reduce the working comfort of the personnel performing the vacuuming operation.
There therefore remains a need for further improving the phthalic anhydride process to improve its operating stability and to increase its capacity. The present invention is concerned with this problem. The invention is further concerned with improving the industrial hygiene conditions and the working comfort of the personnel involved in the catalyst change out from the reactor. The invention is further concerned with improving the recovery and the reuse of the vanadium metal and of the inert support particles from the spent catalyst of the PAN process.
The present invention aims to obviate or at least mitigate the above described problem and/or to provide improvements generally.